Most of Helmand, the biggest poppy-growing province in the country, is already estimated to be under Taliban control with Lashkar Gah - one of the last government-held enclaves - also at risk of falling to the armed group.

About 11,500 civilians, of whom one-third were children, were killed or wounded in Afghanistan in 2016, according to the United Nations.

Afghan security forces now control less than 60 percent of the country, according to US estimates, with the Taliban holding about 10 percent and the remainder contested between various armed groups.